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Democritus Theorized: 1) matter is not infinitely divisible 2) matter is composed to tiny particles called "atomos" 3) atoms cannot be created, destroyed, or ? Aristotle Theorized: 1) empty space does not exist 2) matter is made up of earth, fire, air, or water Dalton Theorized: 1) all matter is composed of extremely small, indivisible, and indestructible particles called atoms (rejected- divided into electron, neutron, proton) 2) atoms of a given element are identical in size, mass, and chemical properties (rejected- isotopes) 3) atoms of a specific element are different from those of another element 4) atoms cannot be created or destroyed (law of conservation of mass) 5) different atoms combine in simple whole number ratios to form chemical compounds (law of definite proportions) 6) in chemical reactions, oms are combined, separated, or rearranged Dalton's atomic model cannonball model Atom the smallest particle of an element that retains the chemical properties of that element Nucleus small region at the center of the atom that contains proton(s) and neutron(s) Electron cloud / shell a relatively large region surrounding the nucleus occupied by negatively charged particles J.J. Thompson concluded that all cathode rays are composed of identical, negatively charged particles J.J. Thompson's atomic model plum pudding model Robert A. Millikan discovered that the electron has almost no mass Ernest Rutherford Gold Foil Experiment conclusions: 1) atom is mostly empty space 2) there is a tiny, dense center of a particle that is positively charged James Chadwick discovered the neutron Intra-nuclear forces forces that old the nuclear particles together Niels Bohr believed the electrons travel on fixed orbits around the nucleus similar to how our planets orbit the sun Niels Bohr's atomic model Planetary model atomic number number of protons in an atom's nucleus isotopes atoms of the same element that have different masses because they have different numbers of neutrons mass number total number of protons and neutrons in the nucleus of an isotope nuclide the general tern for any isotope of any element amu atomic mass unit average atomic mass the weighted average of all known isotopes of an element Avogadro's number 6.02 x 10²³ mole (mol) the SI base unit for the amount of a substance